It is well known that low frequency speakers, usually called woofers, must be specially enclosed to prevent the out-of-phase sound waves from the back of the speaker cone from destructively interfering with the sound waves generated at the front of the cone. Various reflex enclosures have been designed to accomplish that. In some cases, ports are engineered into the speaker enclosures, with the volume of the port designed to tune the volume of the speaker enclosure. A popular type of port is the straight cylindrical port, which is cut to length to achieve a given volume, and fixed in an aperture in the enclosure. A ported system is tuned to a particular resonance frequency, at which the mass of air in the port reacts with the volume of air in the cabinet to create resonance at that tuned frequency. A ported system exhibits improved sensitivity at the resonance frequency, thereby minimizing distortion not only at the tuned frequency, but in the band around that tuned frequency. The result is improved sensitivity at port resonance, and often an extension of the lower cutoff frequency of the loudspeaker.
A conventional cylindrical port can cause audible air turbulence, which is heard as a hissing or rushing sound. The turbulence is caused by the sharp discontinuity at the edges of the port where the port transitions into the volume of the room on one side, or the volume of the speaker enclosure on the other side. The very desirable feature of straight cylindrical ports, however, is that they can be easily tailored to the volume of the enclosure. It is a simple matter to calculate using available nomograms or available computer programs, the length of a port of given cross-sectional area needed to tune a speaker enclosure of particular enclosed volume, and then to simply cut the port to that length. The port is then rigidly affixed into the enclosure, and the job is completed.
In order to minimize air turbulence, it is preferable to utilize flared sections at the ends of the port. The flares avoid the sharp discontinuity of the straight cylindrical port, and thus minimize air turbulence. However, the flared port is non-uniform in cross-section, and it is no longer a simple matter to cut the port to length to tune it to a particular speaker enclosure.
Kits have become available utilizing plastic flared pieces and a central cardboard tube, with the practice being to cut the cardboard tube to a particular length, glue it to the flares, thereby to provide a tuned flared port of predetermined configuration. However, such kits have their own drawbacks.
First of all, using available nomograms or computer programs, and knowing the size of a speaker enclosure and the model of speaker within it, it is possible to determine the exact volume of air within the speaker enclosure. Having selected a frequency at which it is desired to tune the speaker enclosure, it is straightforward to determine the desired length of the port to within 1/8". However, using the plastic and cardboard pieces available heretofore, it is difficult to reliably cut and glue the pieces to achieve the accuracy which the calculation would tend to suggest is desirable. Thus, speaker enclosures might be tuned using the system, but would be tuned to somewhat different frequencies, depending on the actual lengths which can be achieved.
The inaccuracies are due in part to the cutting tolerances achievable with a non-rigid cardboard tube which might be on the order of 4" in diameter, as well as the inaccuracies which are achieved by gluing the tube to flared sections without sufficient fixturing to assure that every glued joint is identical.
The non-rigidity of the final assembly can also be a problem. In very large speaker systems, large masses of air are moved by the woofer, and a cardboard tube suspending a plastic flare has the possibility of mechanical failure over time.
It is also becoming more and more popular to provide high quality speaker systems in automobiles and vans. Those systems have not only the vibrational environment which comes from the speaker system, but also the increased shock and vibration attributable to the automotive application.
In summary, it can be seen that straight cylindrical ports achieve the benefit of simplicity, but do so at the expense of air turbulence. The prior art has attempted to deal with the air turbulence problem, but in doing so has created a system which is difficult to precisely size and also has problems with mechanical stability.